I do a lot of my drawing in Autodesk 123D Design. Mostly because it's free, partly because it's simple to use. One problem I've noticed is drawing threads for nuts, bolts, screws, etc. is not easy. So here is the method that I have found works best for me.

## Step 1: Draw Your Base Shape

The size and shape of your threads will be determined by the base shape you start with. The first image is the shape I use most often which yields a simple thread with a triangular cross section.

The size of your bolt and threads will be determined by the lengths you give to A and B in the second image. Distance A will become the major diameter of your bolt. Distance B will be the minor Diameter. These are B and F respectively in the example image I've included 3rd.

## Step 2: Start Building Your Bolt

The Pitch or distance between the threads will be set in this step See. To do so you'll start by selecting your base shape and Copy/Pasting it the same spot. (Image 1)

Next you're going to move the shape into place. Start by moving it up the Z axis. (Image 2) The distance that you raise the shape should be 1/9th of your pitch. I went with 1mm so my pitch will end up being 9mm, not terribly practical but it gives you the idea. The reason you raise 1/9th is that there will be 9 steps in this half revolution of your threads.

Rotate the shape you've raised 20 degrees. (Image 3)

Repeat these steps 8 more times. Each time copy/pasting the highest shape, raising it up, and rotating it. Be sure to raise and rotate the same amount each step or your threads will come out wonky.

When all this is done your top and bottom shapes will line up when views from above.

## Step 3: Loft

In this step you're going to create the actual threads. We've made it.

Select all the shapes you've created. Be sure to select them in order from bottom to top. (Image 1)

Now, loft them. (Image 2)

## Step 4: Draw Minor Diameter Cylinder

On either the top or bottom face (I use the top because it's easier to see) of the threads you've created you now need to draw a circle to use to make the minor diameter of your bolt. In the first image you can see I drew in an extra line to use in centering the minor diameter circle on the threads. The diameter of you circle should be the same as distance B from step 1.

Now, select all the parts of this circle and extrude them, merging the resulting solids. Extrude the total length of you pitch. In this case that distance is 9mm.

Now you have one section of your bolt.

## Step 5: Build Your Bolt

From here you can copy/paste the solid bolt section you have created and raise it up the Z axis to match up with the existing section. (Image 1) Repeat this step as necessary to give you the thread length required for your project. When you're done you can make all your sections into one object by selecting the Merge option. (Image 2)

And you're done. Hope this helps.

<p>Yay! Just rotated the image 90 degrees to make it vertical along minor axis, and used the desired 1/2 thread rise divided by 9 rotations, per your excellent instructions. Had to place a platform under the thread object, to use its center for all my coordinate system manipulations. Added fillets at top and bottom to clean it up. </p>
<p>Oh here we go. Looking like if I rotate the images in Z direction so that rather than flat they are tilted, that that should do the trick. Just a matter of doing the math to get the right thickness at base (1mm). </p>
<p>Oh. Maybe this technique only works if loft is greater than x, so that there is enough pitch between each 20 degree image to get the thickness you want. Mine is a precision thread with only 1mm between threads. Base of thread needs to be 1mm wide, and thread is 0.5mm tall. Maybe technique will not work. <br><br>I have tried to rotate a triangle around a circle but that technique has a problem when it is time to tweak the break in the circle and raise that end triangle up 1mm.... 123D only will allow me to tweak it upward at an angle perpendicular to the top triangle edge, rather than parallel to the inner edge of the triangle (extending up in Z direction).</p><p>So...stuck on how to make a triangular blessed thread. </p>
<p>Hmmm, I have not worked with loft before. I do it all and get the lofted thread, except it comes out as a 2D (flat) spiral instead of having triangular thread cut. What happened? </p>
<p>Well done! You have solved a problem I have been puzzling with. This is a really clever way to think of it, I had been trying to rotate triangles round the cylinder. You have created a 2-start thread. If you want to fit with existing threads 1-start is going to be more suitable, and a much shallower pitch, but I think the same technique would work with a single point shape.</p><p>The way I did it in the end was to get someone to send me STL files of a male and female thread. I imported them into 123D and edited away the ends I didn't need, then merged them into my designs. I found by increasing the size of the female thread in X and Y (but not Z) by 5% I could print threads that screw together nicely. </p>